Adhesion between the electrode material and the separator and between electrodes and the respective output conductor sheet is of particular importance to the functionality of electrochemical elements. However, electrochemically, loss of content can also occur due to a simple breakdown in content as a result of the electrodes swelling in the electrolyte, or by gassing as a consequence of decomposition. Laminated cells are advantageous in this case, since no spontaneous loss of contact due to gassing occurs in such cells, and the form factor allows a high energy density to be achieved. As a result of the process used to produce it, a laminate is also more resistant to swelling.
The use of polymer electrolytes composed of a film of a copolymer of polyvinylidenefluoride-hexafluoropropylene (PVDF-HFP) and a lithium salt distributed as a conductive salt in it is disclosed in U.S. Pat. No. 5,296,318. The proportion of HFP in the copolymer should accordingly be between 8 and 25 percent by weight.
U.S. Pat. No. 5,460,904 discloses lithium-intercalating electrodes, in which the active substance is distributed in a matrix composed of polyvinylidendefluoride-hexafluoropropylene copolymer. A gel separator, produced from the same materials, is arranged between these electrodes, containing a softener, in particular dibutylphthalate, which is then dissolved out. The proportion of HFP and the gel electrolyte and in the electrode sheets is in each case between 8 and 25 percent by weight.
This method for producing cells has the disadvantage that the softener must be dissolved out. Furthermore, it results in a very soft separator, which can easily be laminated through during the production process, leading to short circuits in the cell. Thicker separator layers are thus required. This reduces the energy density. Furthermore, the proportion of SiO2 in the separator is not stable in the long term, in conjunction with lithium electrodes.
It would, therefore, be advantageous to provide a method for producing electrode sheets of the type mentioned initially, which can be carried out easily, in which good contact is provided between the separator material and the electrode material, and which can be assembled to form cells which have high energy densities.